The present invention relates to the transformation or modification of dispersion in optical fiber transmission systems.
In single mode fibers, the delay time and hence dispersion, depends on wavelength. In general, light sources such as laser diodes have multiple wavelengths, and these different wavelengths travel at different speeds when propagating through a single-mode fiber. This leads to pulse broadening which depends on the source spectral width, the fiber dispersion and the length of the fiber. This pulse broadening can cause power to spread from one bit into an adjacent bit in a bit stream, an occurrence that can cause an error. The bit rate is therefore limited to that value beyond which bit error ratio is unacceptable. Initially, the procedure to overcome this problem was to put limitations on (a) the fiber dispersion, (b) the source wavelength and its spectral width and (c) the product of the bit rate times the repeater span length. These limitations can be relaxed if the delay differences introduced in the system can be compensated.
A system for compensating for dispersion in a single-mode system is disclosed in French Patent Publication No. 2,535,555. That system comprises a monochromator, an array of optical fiber delay lines and an optical signal detector. In a conventional manner, an optical signal generated by a source such as a laser diode is coupled to and transmitted through a single-mode transmission fiber. The monochromator receives the output light from the optical transmission fiber and spatially separates each longitudinal mode of the source. Each mode is injected into one of the short multimode fibers of the delay line array. The length of each short fiber is adjusted to compensate exactly the corresponding delay time induced in the link by its total chromatic dispersion. The ends of the short, delay line fibers converge on the photodetector.
The aforementioned compensator exhibits a dead space between adjacent cores of the delay line array. The multimode fibers of the delay line array include a layer of cladding material that constitutes a substantial part of the fiber. In addition, the fibers are of circular geometry. When such fibers are employed in the delay line array, a substantial part of the light directed thereon goes uncollected. For dispersion compensator applications, this type of array causes higher insertion loss; and more importantly, it causes additional bit errors in digital telecommunications applications. For example, in systems operating with Fabry-perot laser sources, mode partition noise is ever present. In such a system, if the mode containing a substantial part of the bit-energy falls on the cladding, information is lost. Even if the array is originally aligned to capture all of the laser modes, bit errors can occur if there is frequency shift due to temperature or chirping effects. By minimizing the cladding thickness, such adverse effects can be minimized but not eliminated.
It is not always desirable to minimize dispersion and maximize bandwidth. For example, a customer may purchase the sole use of a single-mode optical fiber transmission line and connect its own terminal equipment thereto. The cost of using the transmission line may be based on the maximum data rate that the customer intends to transmit. The bandwidth of the single-mode fiber transmission line may be much greater than that currently needed by the customer. A dispersion transformer similar to that disclosed in the aforementioned French Patent publication No. 2,535,555 can be employed to limit the bandwidth of the transmission line to that bandwidth for which the customer pays. This can be accomplished by utilizing the fiber delay line array to delay certain wavelengths received from the monochromator with respect to other wavelengths received therefrom so that the output pulse from the dispersion transformer is wider than the input thereto.
The term "dispersion transformer" as used herein refers to those systems which minimize or decrease dispersion as well as those which increase dispersion.